Toughening of glass

ABSTRACT

GLASS HEATED TO A TEMPERATURE NEAR ITS SOFTENING POINT IS TOUGHENED BY HEAT EXCHANGE WITH AN OIL HELD AT A TEMPERATURE BELOW THE FLASH POINT OF THE OIL, THERE BEING MAINTAINED IN THE OIL A PROPORTION FROM 0.01% TO 0.07% BY WEIGHT OF A LIQUID HAVING A BOILING POINT LOWER THAN THE FLASH POINT OF THE OIL.

United States Patent US. Cl. '65-116 12 Claims ABSTRACT OF THEDISCLOSURE Glass heated to a temperature near its softening point istoughened by heat exchange with an oil held at a temperature below theflash point of the oil, there being maintained in the oil a proportionfrom 0.01% to 0.07% by .weight of a liquid having a boiling point lowerthan the flash point of the oil.

BACKGROUND oF TnEmvEisnoN This invention relates to the toughening of"glass, for example, in the manufacture of glass panels to be-embodiedin supersonic aircraft. I

The invention is based on the discovery that the efficacy of some oilsas a chilling liquid in the toughening of glass of thickness within thepreferred range, 4 mm. to 12 mm., is improved if there is present in theoil a small regulated proportion of a liquid having a boiling pointlower than the flash point of the oil. For example, the low boilingpoint liquid may bean organic liquid such as carbon tetrachloride,methanol, benzene, toluene, trimethyl alcohol, ethylalcohol, acetone orxylene.

SUMMARY A method of toughening glass, in which the glass is heated to atemperature near to its softening point, comprises toughening the heatedglass by heat exchange with an oil held at a temperature below the flashpoint ofthe oil in which there is maintained a proportion from 0.01% to0.07% by weight of a liquid having a boiling point lower than the flashpoint of the oil. I

Preferably the oil/low boiling point liquid mixture is held at atemperature lying between the boiling point of the low boiling pointliquid and the flash point of the oil.

The oil may be an oil selected from oils having a viscosity in the rangeof 300 to 1000 centistokes at 38 C. and a flash point in the range 220C. to 310 C.

The method of the invention has been devised particularly for thetoughening of sheets of soda-lime-silica glass which are 6 mm. thickwith the intention of use of that glass in the manufacture of glasspanels for supersonic aircraft. The glass of this thickness, as of otherthicknesses within the specified range, can be produced by the floatprocess and in, toughening float glass of this thickness, saidproportion of low boiling point liquid is preferably in the range 0.015%to 0.05% by weight.

The invention also comprehends a toughened glass article ofsoda-lime-silica glass whose thickness is in the range 4 mm. to 12 mm.,having a central tensile stress in the range 500 kg./cm. to 1200 kg./cm.and a ratio of surface compressive stress to central tensile stress inthe range 2:1 to 4:1.

The article may have a thickness of 6 mm., and a central tensile stressin the range 630 kg./cm. to 840 kg./cm.

The invention further comprehends a laminated glass assembly includingat least one sheet of toughened glass as described above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS "ice Glass sheets for example,square glass panels of sodalime-silica glass, which are 30 cm. square,were toughened by the method of the invention. The sheets of glass werefirst subjected to an edge finishing operation in order to produce abright edge finish. This was effected using first a linisher belt coatedwith carborundum followed by a finishing operation with a cork linisherbelt impregnated with ceri-rouge. It was found that with effective edgefinishing of this nature the glass temperature might be maintained inthe lower region of the specified range of 650 C. to 740 C. but it waspreferred, as in a number of examples set out below, to heat the glassto a temperature of about 700 C. prior to quenching.

The glass sheets were then suspended in tongs in a vertical heatingfurnace and when the glass had reached the desired initial temperature,e.g. of about 700 0., the suspended hot glass sheets were lowered at therate of about 30 cm. per second through a mouth in the bottom of thefurnace and into a tank of a selected chilling oil disposed near saidmouth, the oil having a high initial boiling point and containing aselected proportion of a liquid having a boiling point lower than theflash point of the oil.

This selected oil/low boiling point liquid mixture was maintained, ineach example, at a temperature in the range C. to 240 C. The lower thetemperature of the mixture, that is near the 150 C. end of the specifiedrange, the better was the retention of the low boiling point liquid inthe mixture, but at temperatures for the mixture, such as about 200 C.or higher, which were employed in many of the examples, the selectedproportion of low'boiling point liquid was maintained in the mixture byadding further low boiling point liquid as the mixture was circulatedthrough a heat exchanger which maintained the circulation in the tank ofoil/low boiling point liquid mixture at the required temperature of themixture.

Commercial oils were selected for the quenching and a range of oilshaving a viscosity in the range 300 to 1000 centistokes at 38 C. and aflash point in the range 220 C. to 310 C. were selected for use.

. In'a number of the examples, the selected oil/low boiling point liquidmixture contained as selected proportion of carbon tetrachloride (CCl asthe low boiling point liquid, the selected proportion being maintainedwithin the range 0.01% to 0.07% by weight. However, in other examples, aselected proportion of toluene (C H .CH benzene (C H acetone ((CH C0),or xylene (C H (CH was present in the oil, instead of carbontetrachloride. Methanol is also another suitable low boiling pointliquid.

The percentage of the low boiling point liquid necessary depended on thenature of the oil, principally its viscosity, the thickness of the glassto be toughened, and the modulus of rupture, the central tensile stressand the ratio of surface compressive stress to central tensile, requiredin the glass. The glass undergoes a rapid chilling as it is quenched inthe oil/ low boiling point liquid mixture and thereafter cools graduallyto the temperature of the oil/ low boiling point liquid mixture, havingattained the desired stress characteristics by the time it reaches thattemperature. The glass was then removed from the oil/low boiling pointliquid mixture and washed. I v If desired, the toughened glass sheet maybe laminated with one ormore other glass sheets to form, for example, aglass panel to be embodied in supersonic aircraft.

The table quoted below gives a number of examples of thestressesobtained by quenching a glass sheet in a seste oil nt n a ssqifi point liquid.

Difierent low boiling point liquids were employed, and the selectedproportion of said liquid, the thickness of the glass, theinitialtemperatureof the glass and the temperature of the selected'oil/low boiling point liquid mixture were varied to both extremes ofthe particular ranges quoted above; j g a "The selected oils.comprisedfthree dilferent oils and the one -injeach example isgiveninthe table.'The first oil was CyIreXfZOOM. (Mobilv Oil Company), a heavynaphthenic "oil whosellash point -is 305 C and whose viscosity is 974cehtistokes, atl38 C. 'Ihefhe'xt oil was Cylr'e'x FM (Mobil'TOilCompany) 'whichis. a lighter oil a ilash'pbintoof '275 3.7.5114 .aviscosity of 640 sena or at 38,C' 'I'hethird'QiL-Vacuoline (Mobil iiroprt e etale bp lin Finally, the surface compressiye. stressoftheglassshee t, for calculation of the ratio of the surface compressivestress to central tensile stress, is obtained by using the differentialsurface refractometer designed by the Pittsburgh Plate Glass Company.With this instrument, a beam of polarised light is-refracted intoaprincipal surface of the glass sheet at a critical angle which ispredetermined byqthe relative refractive indices of the instrumentcomponents and the glass sheet; the beam traveling'closel to andparallel with. the'surface of thei sh'e'ets'I'heicompressivestress "inthe surface of the sheet is related to, the'difference in the'refractive indices for light which-is polarised perpendicular andparallel *to'the' plane of incidence,,and

' the'light' which emerges from the surface "is ari alysed to TABLE OE"EXAMBLES i, Proportionof Tempera- Thick Tempera lowboilin ture ofModulus 1 Central face compresness of, ture of Type of oil point liq doil/liquid of ruptensile sive stress glass. g1 (Mobile Oil in percentmixture ture in stress in to central inmm. 1n C. Company) by weight I1x190. kgJem. kgJem. tensilestress 4 650,,Vacuoline AA. 0.01 C014 2001,500 500 "2:1 4" 700 Cylrex FM 0.07 0014 200 3, 500 840 3. 5:

. 4 675, Oylrax 200M- 0.07 0614 225 3,250" 850 -3: 5 180 g 2,300 g 6502.6:

. 5 180 2,400 "740 a 2.3: 5- 170 2,500 700 2.6: .51 170 2,550- -.700 I f2:6: 5' 175 I 2,250 7 2.3: 5.. v 180 "2,050"! 1625 -21: 5 170 2,300 680I 2.4: 5 Y 170 2,200 I 650 s 2.3: 5, 175 2,350 700 2.3: 5 170 2,350 2.4:6 200 3,500 840 3.5- 6 200 3,150 700 3.5 6 200 2,800 630 r 3.5- 6 2003,640 840 4 6 f v 200 2,800 630 l 3.5: 6- 700 Vaeuoline-AA. 0.05.0014...200 3,500 840- 3.5:

t 6 670 Cylrex FM;; 0.04 toluene- .170 2, 150 720 2: 6: do.. 0.04benzene170 1,950 090 '2: 6 -do o 170 1,950 700 2: 6 668 --.do 0.03 acetone 1752,050 660 2.1: 6. 683.-...gdo 0.02xylene 170 2,150 720 D 2: ,8 v 675Vaeuoline AA 0.01 0014 175 2,200 590 3: -8 200 4,000 900 3.5:

123 670 200 0.07 Y 240 5,500 Q 1,100 4: 12 660, Oylrex FM 0.015 001 240g 4,000 1 950, v 3.5- 12 s 740 -Vaeuo11ne AA. 0.07 0014....'.. 150 b 4,500, 1,200 3. 5

Oil: Company) is a lighter oil still whose flash poiu tfis thus-providesan advantageousmethod 225, C. and viscosity is inthe range I 310 to 3fl2centistokes at 38 C. u

. After each glass sheet was toughened, themodulusftff rupture, thecentral tensile stress, and theratio of surface compressive stress tocentral tensile stress,'were deter mined in the following manner. 7 j

The toughened glass sheet under test was placed across a. pair of .knifeedges and a steadily increasing load was applied through a similar pairof knifeedgesfplaced near .the ,mid-point of the glass sheet.l'Ihemodulu sof rupture, yvhich elfectively. corresponds to the breaking's'tressin the convex surface of the glass sheet when in tension, wasthen calculated from the load applied at the instarit of fracture andthe cross-section of the glass sheet.

The-central tensile stress in the toughenedfglass sheet was; measuredby.an instrument developed by Triplex Safety Glass Company Limited which isemployedto pass a beam of polarised light at a grazing angle into a,'pr'inciu the fringe formed in the beam and'com- 'paringthis slope witha previous calibration.

tensile stress in the glass sheet is then obtained by observ- .ing'theslope of of toughening glass so. thatithas a modulus of rupture that maybe 'as'high as 5500 kg./cm.=, and a ratio of sur- -fao'e'- compressive:stress to central tensile stress thatm'ay be as high .as 4zl,withoutiimpairing the optical. charac- -teristics of"the glass.Thistoughened glass is particularly advantageous iniapplications' wherehigh strength isrequired, and the fact that the glass break into smallfragments if fractured is of little consequence. Thus, for example, theglass is particularly.v efiective in the manufacture of panels forsupersonic aircraft, and other applicationafor example in domesticbuilding where such high strength isrequired. v f e We claim:

'{ LA method of=tougheningglass having 'a'thickn'ess of 2 liquid beingmaintained in the oil bath in selected proportion from 0.01% to 0.07% byweight and having a boiling point sufficiently lower than the flashpoint of the oil to produce said rapid chilling of the glass immediatelyit is quenched.

2. A method according to claim 1, wherein the oil/low boiling pointliquid mixture is held at a temperature lying between the boiling pointof the low boiling point liquid and the flash point of the oil, and saidproportion of the low boiling point liquid is maintained by addingfurther low boiling point liquid to the oil/ low boiling point liquidmixture. 4

3. A method according to claim 1, wherein the oil is selected from oilshaving a viscosity in the range 300 to 1000 centistokes at 38 C and aflash point in the range 220 C. to 310 C.

4. A method according to claim 1, wherein the low boiling point liquidis selected from carbon tetrachloride, toluene, benzene, acetone andxylene.

5. A method according to claim 1, wherein there is maintained in theoil/low boiling point liquid mixture a proportion from 0.01% to 0.05% ofsaid low boiling point liquid.

6. A method according to claim 1, wherein the temperature of the oil/lowboiling point liquid mixture is in the range 150 C. to 240 C., and saidmixture induces in the glass a central tensile stress in the range 500kg./cm. to 1200 kg./cm.'-' and a ratio of surface compressive stress tocentral tensile stress in the range 2:1 to 4:1.

7. A method according to claim 6, wherein the glass is heated to atemperature in the range 650' C. to 740 C.

8. A method according to claim 7 of toughening a sheet ofsoda-lime-silica glass which is 6 mm. thick, wherein said proportion oflow boiling point liquid is in the range 0.015% to 0.05% by weight.

9. A method according to claim 8, wherein said oil/low boiling pointliquid mixture induces in the glass at central tensile stress in therange 630 kg/cm. to 840 kg./cm.

10. A method according to claim 1, wherein the temperature of the oil/low boiling point liquid mixture is in the range 150 C. to 200 C., andwherein said mixture induces in the glass a central tensile stress inthe range 500 kg./cm. to 840 kg./crn. and a ratio of surface compressivestress to central tensile stress in the range 2:1 to 4:1.

11. A method according to claim 10, wherein the glass is heated to atemperature in the range 685 C. to 720 C.

12. A method according to claim 1, of toughening a sheet of soda-limesilica glass which has a thickness in the range 4 mm. to 8 mm., whereinsaid oil/low boiling point liquid mixture induces in the glass a centraltensile stress in the range 500 kg./cm. to 900 lrg./cm. and a ratio ofsurface compressive stress to central tensile stress in the range 2:1 to4:1.

References Cited UNITED STATES PATENTS 3,186,816 6/1965 Wartenberg -l 163,271,207 9/ 1966 Davis 65-1 16 X FOREIGN PATENTS 316,108 11/ 1956Switzerland 651l6 ARTHUR D. KELLOGG, Primary Examiner

